1. Field of the Invention
The present invention relates to a boundary line detecting method for specifying, by image processing, a boundary line between areas different in reflected light intensity, as well as a positioning method and apparatus for positioning, using the detecting method, for example a magnetic head body for a hard disk device and a support member such as a load beam relative to each other.
2. Description of the Prior Art
FIG. 8A is a plan view showing a conventional magnetic head positioning apparatus and FIG. 8B is a side view thereof.
A magnetic head body 1, which is for a hard disk device, comprises a slider and a recording portion and a reproducing portion both of a thin film structure disposed at a trailing-side end portion of the slider. A load beam 2 as a support member for supporting the head body 1 is formed using a plate spring material. At a tip portion of the load beam 2 the head body 1 is supported through a thin plate spring called flexure. A pivot 3 is formed in the shape of a concave sphere at the tip of the load beam 2 and its apex is in spot contact with the upper surface of the head body. The head body 1 is supported pivotably in both rolling and pitching directions with the apex of the pivot 3 as fulcrum.
In the conventional positioning process for the head body 1 and the load beam 2, two side faces of the slider of the head body 1 are positioned and held while being pressed against stepped portions 4a and 4b which are formed perpendicularly to each other on the upper surface of a carrier 4, the carrier 4 being moved in the direction of arrow L at each step of the so positioning process. The carrier 4 with the head body 1 held thereon is located at a predetermined step position between carrier positioning blocks 5, 5, at which position the load beam 2 is installed on the carrier 4.
A pair of positioning pins 4c and 4d are implanted in the upper surface of the carrier 4, and positioning holes 2a and 2b formed in the load beam 2 are fitted on the positioning pins 4c and 4d, whereby the load beam 2 is positioned on the carrier 4. The load beam 2 is held with a jig in the thus positioned state on the carrier 4. In this state the flexure provided at the tip of the load beam 2 and the head body 1 are bonded and fixed together.
In the magnetic head of this type, the relative position between the head body 1 and the pivot 3 exerts a great influence on a floating posture of the head body on a recording medium such as a hard disk. However, the positioning method using the positioning apparatus shown in FIG. 8 has encountered a limit in determining a relative position between the head body 1 and the pivot 3 with a high accuracy.
More particularly, the position where the load beam 2 is to be installed is determined on the basis of the positioning holes 2a and 2b. But a machining tolerance in the relative position between the positioning holes 2a, 2b and the pivot 3 gives rise to an error in the position of the pivot 3 on the carrier 4. The head body 1 is positioned on the basis of the stepped portions 4a and 4b on the carrier 4 and therefore, as to the relative position of the pivot 3 and the head body 1, not only the aforesaid machining tolerance but also positional dimension tolerances between the stepped portions 4a, 4b and the positioning pins 4c, 4d of the carrier 4, as well as fitting clearance tolerances between the positioning pins 4c, 4d and the positioning holes 2a, 2b, are accumulated.
As a result, a maximum of about .+-.20 .mu.m tolerance occurs between a designed abutment position of the pivot 3 on the head body 1 and an actual position where the pivot 3 abuts the head body 1. When the head body 1 assumes a floating posture on a recording medium such as a hard disk, the above .+-.20 .mu.m error of the pivot position causes a difference of about .+-.7.8 nm in terms of a floating distance in the rolling direction and a difference of about .+-.1.6 nm in terms of a floating distance in the pitching direction.
Further, when the assembly of the head body 1 and the load beam 2 is incorporated in a hard disk device for example, a tolerance of about .+-.7.6 nm occurs in the floating distance of the head body 1 due to variations in static posture of the head body 1 on the recording medium or in the spring pressure of the load beam.
If the variation in the floating distance caused by the positioning error between the head body 1 and the load beam and the variation in the floating distance caused by variations in static posture or spring pressure are merely added together, the result obtained becomes very large, thus causing defective products whose variations in the floating distance of head body 1 exceed an allowable value. Consequently, the percentage defect becomes high.
Recently, with an increase in recording density, the slider of the head body 1 has become smaller in size and the floating distance of the head body has become shorter, resulting in the tolerance thereof becoming narrower. Therefore, it is necessary that the control of the floating distance be done with a high accuracy.
When attention is paid to variation factors in the floating distance, the attempt to minimize the variation in the floating distance caused by variations in static posture or spring pressure is restricted by the entire structure of the head, so in order to realize such attempt it is necessary that the relative position between the head body 1 and the pivot 3 formed in the load beam 2 be determined with a high accuracy.
As a method for determining a relative position between the head body 1 and the pivot 3, reference is here made to a method in which the bonded portion of the load beam 2 and the head body 1 and the vicinity thereof are photographed on a larger scale with a camera, then the distance between an edge portion of the slider of the head body and a central position of the pivot is determined on the image thus obtained, and a check is made to see if the distance is within the as tolerance or not.
However, with such an image photographed by a camera, it is impossible to observe distances shorter than the arrangement pitch of photodetectors such as CCDs. For example, even if an attempt is made to specify an edge portion of the slider of the head body 1, it is impossible to specify its position at a distance shorter than the arrangement pitch of the photodetectors.